Why is my garage or outbuilding damp?

What this usually means

A garage or outbuilding is, in moisture terms, the opposite of a home: it is usually unheated, so its surfaces stay cold; uninsulated, so those surfaces are as cold as the outdoors; and unventilated, so any moisture that gets in tends to stay. When relatively warm, moist air — from outside on a mild damp day, from a wet car, or from the ground — meets the cold concrete floor, the cold walls and the cold underside of a metal or single-skin roof, it condenses, leaving the surfaces wet. This is why a garage can stream with water or feel clammy even with no leak: it is condensation on cold surfaces in a space with no ventilation to clear it.

Ground moisture is the other major source. Many garages and outbuildings have a concrete floor laid directly on the ground without a damp-proof membrane, or solid walls without a damp-proof course, so moisture from the ground rises into the floor and walls and evaporates into the space, keeping it humid. Combined with the lack of ventilation, this moisture has nowhere to go. Penetrating damp — rain getting in through a leaking roof, failed render, gaps around doors, or an exposed wall — can add to it, particularly in older or poorly maintained outbuildings. So the damp in a garage is usually a mix of condensation on cold surfaces, moisture rising from the ground, and sometimes ingress, with one tending to dominate.

Keeping the space dry depends on which source dominates, which is worth establishing rather than guessing. Where condensation is the main problem, the answer is ventilation — providing airflow to carry the moist air out — and, where the space is used, some insulation and gentle heating to keep surfaces above the dew point; simply heating an unventilated, uninsulated garage can even make condensation worse by raising the air's moisture-carrying capacity then letting it condense on the still-cold surfaces. Where ground moisture dominates, addressing the floor (a sealed or membrane-backed finish) and the walls helps. Where there is ingress, the leak or defect must be fixed. An assessment that identifies the dominant source ensures the right measure — usually ventilation first — rather than an ineffective or counterproductive one.

Common causes

Signs and symptoms

An unheated, uninsulated outbuilding tracks the external temperature closely, so its internal surfaces are cold whenever the outdoor air is cold, and condensation occurs whenever air with a given moisture content contacts a surface below its dew point. Because the space has little thermal mass buffering and no heating to lift surface temperatures, mild, humid air — common in spring and autumn — readily condenses on the cold concrete and the cold roof, and the absence of ventilation prevents the humid air being replaced with drier air, so the moisture persists. This is surface condensation in an extreme form, driven by cold surfaces and stagnant humid air rather than by any leak.

Ground moisture adds a continuous vapour source where the construction lacks a barrier. A slab cast directly on the ground without a damp-proof membrane, or walls without a damp-proof course, allow capillary moisture to migrate from the ground into the floor and walls and evaporate into the air, raising the internal humidity and keeping surfaces damp; with no ventilation to remove that vapour, the space stays humid. Penetrating damp through a defective roof, render or door gaps introduces liquid water that compounds the load. The relative contribution of condensation, ground moisture and ingress varies with the construction, the exposure and the use, so the dominant mechanism must be identified to target the remedy.

The remedies follow the mechanism, and ventilation is usually primary. Providing airflow removes humid air and is the most effective and least counterproductive measure for condensation; insulation and gentle heating help only if the space is to be used and surfaces can be kept above the dew point — heating an unventilated, uninsulated space can paradoxically worsen condensation by raising the air's absolute humidity while the surfaces remain cold. Ground moisture is addressed by a sealed or membrane-backed floor finish and wall treatment, and ingress by repairing the defect. An assessment that logs the humidity, maps the cold and wet surfaces and checks the floor and ventilation establishes which source dominates, so the space is kept dry by the right measure — ventilation first in most cases — rather than by heating alone, which often disappoints or backfires.

How to keep a garage or outbuilding dry

Identify whether condensation, ground moisture or ingress dominates, then ventilate first, address the floor and walls for ground moisture, and fix any ingress — adding insulation and heat only where the space is used.

1. 01

   Identify the dominant source

   Establish whether the damp is condensation, ground moisture or ingress.

2. 02

   Provide ventilation

   Add airflow to carry moist air out — usually the first and most important step.

3. 03

   Address ground moisture

   Use a sealed or membrane-backed floor finish and treat the walls where moisture rises.

4. 04

   Fix any ingress

   Repair a leaking roof, failed render or gaps around doors letting rain in.

5. 05

   Insulate and heat if used

   Where the space is used, add insulation and gentle heat to keep surfaces above the dew point.

6. 06

   Keep moisture sources out

   Avoid storing wet items or drying washing in an unventilated space.

How to prevent it coming back

• Ventilate the space to clear moist air.
• Avoid heating an unventilated, uninsulated garage on its own.
• Seal or membrane the floor against ground moisture.
• Keep wet cars, items and washing out where possible.